Electro-optic modulators are used in optical communications systems to rapidly modulate and optical signal in accordance with an electrical signal. In an electro-optic mode converter, a type of electro-optic modulator, an input beam of light having an input state of polarization (SOP) impinges on and traverses through an electro-optic waveguide that is subjected to an applied electric field. The applied electric field modifies the modal properties of the waveguide via the electro-optic effect. When the input SOP is not aligned with a principal axis of the waveguide, and the propagation speed of the light through the two principle axes differs, the beam of light at the output of the waveguide will generally have an output SOP different from the input one. With proper choice of input SOP, waveguide geometry and applied electrical field, it is possible to have the output SOP orthogonal to the input SOP. This allows the use of the electric field to control the mode conversion so that the output optical signal is modulated in accordance with a signal used to generate the electric field.
Structurally, an electro-optic mode converter or a modulator will usually include an electro-optic guiding layer such as a III–V semiconductor or LiNbO3-type material, an optical waveguide defined in the optical guiding layer and an electrode structure disposed in the vicinity of the optical waveguide. As a voltage is applied to the electrodes, an electric field is formed across the optical waveguide and modifies the modal properties of the waveguide such as the orientation of its principal axes and/or its birefringence thus allowing for a modification of the SOP of a light beam traversing the optical waveguide. In the ideal, the principle axes are rotated to 45° from an X-Y orientation, and the birefringence of the axes is then modulated to control the phase relationship of the two fundamental hybrid optical modes. The phase relationship in turn determines the output SOP.
U.S. Pat. No. 5,566,257, hereinafter referred to as '257, issued Oct. 15, 1996 to Jaeger et al., which is incorporated herein by reference, discloses an electro-optic modulator having an electrode structure with two spaced apart conductive strips or electrodes disposed on either side of a single semiconductor optical waveguide. Each electrode includes projections, or fins, projecting towards the other conductive strip and disposed so as to affect the optical permittivity tensor of the waveguide material upon a voltage being applied to the electrodes. At the end of the projections, adjacent the waveguide, are pads, the geometry of which having an impact on the electrode structure capacitance.
The electrode structure of '257 is referred to as a “slow wave” electrode structure because it matches a microwave index to the optical index of the waveguide, so that a signal applied to the electrodes propagates through the signal path at the same velocity that the optical signal propagates through the waveguide. As a result, the optical signal can be cleanly modulated in accordance with the changing electric field generated by the application of a signal to the electrodes.
The teachings of '257 provide an electro-optic modulator requiring less electrical and optical power and capable of running at higher frequency than Mach-Zehnder type slow wave modulators such as described in U.S. Pat. No. 5,150,436, hereinafter referred to as '436, issued Sep. 22, 1992 to Jaeger et al., which is incorporated herein by reference.
For efficient operation, electro-optic mode modulators such as the one disclosed in '257 usually require a bias voltage to adjust the operating point of the modulator. The bias voltage may be applied to the signal electrode with the ground electrode connected to the ground of the package housing the mode converter in order to achieve current return. This method of biasing requires that a DC blocking circuit be disposed at the input of the electrode structure in order to prevent excessive voltage due to the biasing voltage from appearing in the modulation driving circuit. Furthermore, the DC blocking circuit must be such that it does not affect the modulation signal across the operational bandwidth of the modulator.
It would thus be desirable to have a mechanism for applying a biasing voltage to the mode converter that does not require a DC blocking circuit and that does not impair the operational bandwidth of the mode converter.
Additionally, electro-optic modulators as disclosed in '257 tend to have their electrodes exhibit a resistive loss of the electrical signal that increases as the frequency of the electrical signal is increased. This is due to the skin effect and leads to a substantial reduction of the electro-optic bandwidth of the modulator.
Consequently, it would be desirable to have a mechanism for alleviating the frequency dependent skin effect.
Another concern for electro-optic mode converters or modulators such as disclosed in '257 relates to their grounding. In order for the mode converter or modulator to exhibit proper radio frequency (RF) behaviour, the ground electrode of the modulator should be in electrical contact with the ground of the modulator package and, the connection length between the modulator and package grounds should be as short as possible. Conventional methods of accomplishing this connection usually lead to the appearance of mechanical stress in the mode converter as the temperature of the package varies. Such stresses adversely affect the performance of the mode converter, and can induce an unwanted strain-optic effect in the waveguide that changes its known parameters.
It would thus be desirable to have a grounding mechanism that provides proper RF behaviour and does not affect the mode converter performance as the temperature of the package housing the mode converter is varied.